Adding roof windows during a re-roof

Fitting a roof window into a finished roof means lifting tiles from a patch of slope, cutting rafters, trimming in a frame and rebuilding the covering around it. On a roof that is being renewed anyway, much of that is already under way. The covering is off, the underlay is fresh, and the opening is formed before the new battens and tiles go back over it.

Access helps too. MyBuilder's 2026 roof window guide puts scaffold for a stand-alone window job at about £500, and installation at £350 to £600 for standard jobs, labour and fitting. Those are national figures. With scaffold already standing for the re-roof, that access cost is shared. Our quote covers the window as an option you can keep or drop.

A rafter runs from the ridge to the eaves, and a roof window usually interrupts one or two of them. The carpenter cuts the rafters and fixes new timbers across the gap, called trimmers, so the load is carried around the opening. The frame is then fixed to those timbers, and the surrounding covering is formed with a flashing kit made for that window and that covering.

The inside needs attention as well. The opening is lined with insulated board, sealed against the vapour barrier and finished with a splayed reveal that lets daylight spread into the room. If those details are skipped, the window can collect condensation and the cold edge shows up as damp on the plaster.

The detailing at the head and sides, where water would try to run behind the frame, is the part that matters most. Whether the flashing is lead or the maker's kit, it has to lap correctly with the tiles or slates. Underlay is cut and dressed into a tray around the opening so any water that gets in is sent back out. If you are choosing between tile, slate and stone, our guide to choosing a covering for your new roof shows how the covering affects the flashing.

Roof windows fall under Class C of the General Permitted Development Order, and Class C sets limits. Under that class the unit may rise only 0.15 metres from the original slope, and it may not push the highest part of the roof any higher. If the slope faces the side of the house, the glass always has to be obscured, and any opening part must sit more than 1.7 metres above the floor inside, or else the window is fixed shut.

Class C carries no restriction for conservation areas or the Cotswolds National Landscape, so inside those limits a roof window can still be permitted development there. An Article 4 direction or a planning condition can remove that right, though, and a listed building needs its own consent. On a protected house, speak to the planning team at Stroud District Council before ordering. Dormers are a different matter, and are not permitted development on that land.

Under Part L of the Building Regulations (Approved Document L, 2021 edition), a new roof window in an existing home should reach a U-value of 1.4 W/m²K, or achieve Band B on the Window Energy Rating scale, whichever the supplier quotes. That figure measures heat lost through the glazing, so a lower number is better. A rooflight, as that document defines it, has its own limit of 2.2 W/m²K. Ask which product you are being offered and check its figure on the data sheet.

If the window will serve a loft room, the escape rules enter the picture. Approved Document B accepts a window as the way out only when the floor sits no higher than 4.5 m above the ground. The window then needs an openable area of at least 0.33 m², at least 450 mm in height and width, with its bottom edge no more than 1,100 mm above the floor. Where the house has two storeys, a loft room generally also needs a protected stair, which no roof window can replace, so bring building control in early.

Light, heat and glare

Position matters as much as size. A south-facing window floods a loft room with light and summer heat, so ask about blinds, and remember that a window set high on the slope lights the ceiling while one set low gives a view out. Both choices are easier to make before the rafters are cut.

Ask for the window to be covered clearly in the quote: the unit, the flashing kit, the trimming carpentry, internal lining and finishing, and any extra scaffold. Ask for the make and size, and whether it is centre-pivot or top-hung. Should the job be notifiable, we set out the consents involved and make the notification to building control ourselves.

The window work sits inside our 10-year workmanship guarantee, written down, and we remove the waste when the day is done. An opening also changes what leaves the roof, so see who removes the old roof, and where it goes. To see how the window compares with another firm's, our guide to comparing re-roof quotes like for like covers the rest. The roof replacement page links the wider job.